STAPHYLOCOCCI

Mrs. Jincy EaliasM.Sc. (N)Asst. Professor

Mr. Binu BabuMBA, M.Sc. (N)Asst. Professor

Staphylococci

Staphylococcus is a gram positive cocci. It is frequently involved in nosocomial and opportunistic infections. Different species of Staphylococcus are;

• S. aureus – lives in respiratory tract and skin.• S. epidermidis – lives on skin and mucous

membranes. • S. hominis – lives around sweat glands.• S. capitis – live on scalp, face, external ear.• S. saprophyticus – lives on skin, intestine, vagina.

General Characteristics of the Staphylococci

• Common inhabitant of the skin and mucous membranes

• Spherical cells arranged in irregular clusters• Gram-positive • Lack spores and flagella• May have capsules

Staphylococcus aureus

Morphology• Non motile and non spore forming• Gram positive cocci• Size is 0.7 to 1.2 µm in diameter• Irregular clusters that resemble cluster

of grapes

S. Aureus Morphology

Cultural characteristics• Grows in large, round, colonies.• Optimum temperature of 37oC• Facultative anaerobe• Withstands high salt, extremes in pH, and high

temperatures• Isolation is best done in blood agar. Produce a

characteristic golden yellow carotenoid pigment. On blood agar colonies are usually surrounded by a zone of clear hemolysis.

zone of hemolysis

Biochemical properties• Produces many virulence factors. i.e

production of enzymes and toxins

Virulence factors of S. aureusEnzymes:• Coagulase – coagulates plasma and blood.• Hyaluronidase – digests connective tissue• Staphylokinase – digests blood clots• DNase – digests DNA• Lipases – digest oils; enhances colonization

on skin• Penicillinase – inactivates penicillin

Toxins:• Hemolysins – lyse red blood cells• Leukocidin – lyses neutrophils and

macrophages• Enterotoxin – induce gastrointestinal

distress• Exfoliative toxin – separates the epidermis

from the dermis • Toxic shock syndrome toxin (TSST) –

induces fever, vomiting, shock, systemic organ damage

Pathogenesis• Present in most environments frequented

by humans.• Carriage rate for healthy adults is 20-60%.• Carriage is mostly in anterior nares, skin,

nasopharynx and intestine• Predisposition to infection include: poor

hygiene and nutrition, tissue injury, preexisting primary infection, diabetes, immunodeficiency.

Staphylococcal Disease

• Localized cutaneous infections – invade skin through wounds, follicles, or glands.

• Systemic infections – Osteomyelitis – bone infection – Bacteremia – transmission of bacteria from one infected

site to another. Eg: endocarditis

• Toxigenic disease – Food intoxication – ingestion of heat stable enterotoxins;

gastrointestinal distress– Staphylococcal scalded skin syndrome – toxin induces

bright red flush, blisters, then desquamation of the epidermis

– Toxic shock syndrome – toxemia leading to shock and organ failure

Laboratory diagnosis

• Staphylococcus aureus is frequently isolated from samples such as pus, tissue exudates, sputum, urine, and blood.

• Different methods for laboratory diagnosis of Staphylococcus aureus are;Gram stainingCultureCoagulase test

Gram stainingThe diagnosis of staphylococcus aureus is

suggested by the finding of gram positive bacteria in clumps in the sample.Culture

10 ml venous blood is inoculated into 50 ml glucose broth. Identification of staphylococcus aureus depends on its colony characteristics in culture media such as blood agar. The characteristics haemolysis and yellow pigmentation suggest S. aureus.

Coagulase test1 ml of overnight broth suspension from agar

plate is mixed with 1 ml of 10% dilution of human plasma (any fresh plasma). The mixture is incubated at 37oC for 3-6 hrs. Check for presence of clot. If clot appears it is positive and if no clot it is negative.

Treatment

• 95% S. aureus have penicillinase and are resistant to penicillin and ampicillin. Cephalosporins are the drug of choice in case of penicillin resistance.

Prevention of Staphylococcal Infections

• Universal precautions by healthcare providers to prevent nosocomial infections

• Hygiene and cleansing

Thank You